1. Field of the Invention
This invention relates to an optical coupling module, and more particularly to an optical coupling module for coupling optical fibers to each other.
2. Description of the Related Art
When optical fibers are coupled by an optical connector, if optical axes of the optical fibers are displaced from each other in the coupled portion, insertion loss is generated due to radiation of light. Therefore, to prevent the insertion loss from being caused by the displacement of the optical axes, conventionally, coupling of optical fibers has been performed using a split sleeve.
FIG. 9 is a view of the split sleeve. The split sleeve 10a is provided with a slit. The sleeve is a hollow cylindrical part for positioning end faces of optical fibers, and in general, a hollow cylindrical sleeve having a slit formed in the direction of the length thereof is called a “split sleeve”.
FIG. 10 is a view of the appearance of an optical coupler part. An optical connector 6 is formed with a ferrule 3 for holding an optical fiber f1, while an optical device housing 5 is formed with a fiber stub 4 (associated with a ferrule on a receiving side) for holding an optical fiber f2. A split sleeve 10a is housed in a sleeve casing 20a, and the fiber stub 4 is inserted into an inner hole on a right end side of the split sleeve 10a, as viewed in the figure.
In coupling (joining) the optical fiber f1 on the connector side to the optical fiber f2 on the housing side, the ferrule 3 on the connector side is inserted into an inner hole formed on a left end side of the split sleeve 10a, as viewed in the figure.
As the ferrule 3 is inserted, the split sleeve 10a is elastically deformed, causing an end face e1 of the optical fiber f1 to be joined to an end face e2 of the optical fiber f2, such that the optical axes of the optical fibers f1 and f2 are aligned by causing the outer shape of the ferrule 3 to match that of the fiber stub 4. By using the split sleeve 10a described above, it is possible to substantially align the optical axes of the optical fibers f1 and f2 with the alignment effect of the split sleeve 10a obtained even when the positions of the optical axes are different from each other in the order of 100 μm.
As a conventional optical coupling technique, there has been proposed a technique in which a slit fixing portion for fixing the splitting slit of the split sleeve is provided, whereby even when a load in a perpendicular direction acts on the split sleeve, the sleeve is prevented from being deformed by excessive opening of the slit (e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2005-156969 (Paragraph numbers [0022] to [0034], and FIG. 1).
Conventionally, in the optical connector, optical loss generated at coupled end faces of the optical fibers largely varies depending on an external force applied to the optical connector. To avoid this problem, during a work for installing a large-capacity trunk optical communication system, a curing treatment (protective treatment), a forming treatment in which an optical fiber cable is held in a bent state, or a like countermeasure is carried out so as to prevent an external force from being applied to the optical connector.
On the other hand, recently, communication systems, such as routers, have become more sophisticated and faster, and optical interfaces have come to be more often used in small-scale routers. In installing a small-scale router, a simple work for laying an optical fiber is often carried out.
In this case, the protective measure is not sufficient compared with the work for installing the large-capacity trunk optical communication system, so that there is a risk, for example, that excessive load is momentarily applied to an optical fiber, e.g. when a worker carelessly stumbles over the optical fiber. To stabilize the communication even in such a situation, it is required to improve a wiggle characteristic, which is an optical loss-changing characteristic exhibited when an external force is applied to an optical connector or an optical fiber.
FIG. 11 is a view useful in explaining the wiggle characteristic. As an environment for the measurement, the optical connector 6 is held parallel to the ground, and a load fixing portion 71 having a load 72 attached thereto is fitted on an optical fiber f stretched in a direction perpendicular to the ground. Further, an optical power meter 8 is connected to a leading end of the optical fiber f.
In this state, an optical signal is delivered toward the optical power meter 8 through the optical fiber f, and the relationship between the rotational angle of the optical connector 6 and a change in loss of the optical signal is measured while the optical connector 6 is rotated leftward and rightward each through 360°. The result of this measurement provides a value of the wiggle characteristic. It should be noted that as the load 72, there are employed a load of 0.5 pounds for an LC connector, and a load of 1.5 pounds for an SC connector.
Now, the influence of an external force depends on the structure of the optical connector 6. The shape of the structure of the fitting portion of the connector 6 does not have rotational symmetry with respect to the optical axis, and hence yield strength of the optical connector 6a against the external force has a rotational angle characteristic. Further, when the sleeve undergoes elastic deformation caused by the external force, in general, different characteristics are exhibited depending on the direction of rotation of the optical connector 6. Therefore, when the wiggle characteristic is discussed, it is necessary to consider both the angle characteristic of the rightward rotation and that of the leftward rotation. When the wiggle characteristic is degraded at a specific angle, the insertion loss is largely changed when a certain external force is applied in a direction corresponding to the specific angle, which lowers a reception level at a remote station, which can be a cause of communication errors.
FIG. 12 is a view of a shape of the split sleeve 10a formed when an external force is applied to the split sleeve 10a. The conventional split sleeve 10a holds the ferrule 3 with a stress obtained by the elastic deformation of the sleeve, to thereby align an optical axis on the receiving side and an optical axis inside the ferrule 3. At this time, the holding force of the split sleeve 10a per se is weak, and is insufficient to withstand the load 72 (the LC connector: 0.5 pounds, the connector: 1.5 pounds) used for evaluation of the wiggle characteristic.
As a result, when the load 72 described above is applied to the optical connector 6 or the optical fiber f, the conventional split sleeve 10a cannot withstand the external force, so that the slit of the split sleeve 10a is largely opened and the ferrule 3 is displaced in the direction of application of the external force, resulting in displaced optical axes.
FIG. 13 is a view of a state in which the ferrule 3 is displaced. When an external force is applied perpendicularly downward to displace the ferrule 3, an angle θq between an end face of the ferrule 3 and an end face of the fiber stub 4 is increased (θq represents an angle created due to the difference (play) between the inner diameter of the split sleeve 10a and the outer diameter of the ferrule 3). At this time, the split sleeve 10a itself has undergone elastic deformation in the opening direction of the slit.
Now, when the target value of the wiggle characteristic is set to 1.5 dB, it is necessary to hold the width d of displacement of the optical axes not larger than 2 μm (when θq=0 holds), or hold the angle θq not larger than 0.36 degrees (when d=0 holds), according to the results of analysis of a model formed by Gaussian approximation of a propagation mode of the optical fiber.
In the conventional split sleeve 10a, however, changes in d and θq are large, so that generally, the value of the wiggle characteristic exceeds 10 db. Therefore, to prevent the elastic deformation in which the slit is opened, conventionally, a precision sleeve is sometimes employed which has an inner diameter machined more precisely than the inner diameter of the normal split sleeve 10a. 
The precision sleeve has the inner diameter thereof precisely machined such that a gap between an inner surface of the sleeve and an outer surface of the ferrule 3 is as small as approximately several μm. In the LC connector, for example, with respect to the outer diameter φ of the ferrule in a range between 1.2485 mm and 1.2495 mm, the inner diameter φ of the precision sleeve is in a range between 1.251 mm and 1.252 mm.
The changes in d and θq of the precision sleeve are smaller than the changes in d and θq of the split sleeve 10a, so that the wiggle characteristic is improved to a certain degree compared with the split sleeve 10a. However, although the precision sleeve is precisely machined with respect to the inner diameter of the ferrule 3, it cannot be said that the precision sleeve is precisely machined with respect to the outer diameter of the fiber stub 4. Further, the precision sleeve suffers from the problem that “the tilt (inclination) of the sleeve” is caused by the difference between machining accuracies of individual fiber stubs.
FIG. 14 is a view of the tilt of a sleeve. When an external force is applied to a precision sleeve 10a-1, the precision sleeve 10a-1 is tilted through an angle of θs at the maximum due to a gap between the inner surface of the precision sleeve 10a-1 and the outer surface of the fiber stub 4 (an inclination angle θs is formed between the precision sleeve 10a-1 and the sleeve casing 20a).
Further, the tilt of the precision sleeve 10a-1 sometimes causes the inclination or deformation of the fiber stub 4. Such a state causes degradation of the wiggle characteristic, and hence it is difficult to improve the wiggle characteristic simply by employing the precision sleeve 10a-1 (it is difficult to achieve the target value 1.5 dB of the wiggle characteristic). Furthermore, even if the precision sleeve 10a-1 is employed, the play of the ferrule 3, defined by the angular difference between the optical axes, cannot be sufficiently suppressed.
As described above, the conventional split sleeve 10a and the precision sleeve 10a-1 are required to have a structure capable of preventing excessive elastic deformation or a structure capable of preventing the tilt of the sleeve. To improve communication quality, further improvement measures are strongly demanded.